This invention relates to wooden I-beams suitable for use as joists and rafters in construction of frame buildings, and particularly to wooden I-beams having solid lumber flanges and plywood webs.
Where strength is required to support loads applied primarily in one direction, considerable saving of weight and material cost may be accomplished by using a beam arranged in a form having a cross section resembling the capital letter "I" and called an I-beam. This general design provides required strength to support compressive and tension loads generated by application of a load to the top of such an I-beam resting on supporting structure, as when such a beam is used as a rafter or joist.
When a downward load is applied to the top of a beam so constructed, with the ends of the I-beam supported from below and the "I" cross section in an upright orientation, the stresses generated are distributed as compression along a top flange portion of the I-beam and tension in a bottom flange. The width of the top and bottom flanges gives required lateral stiffness, allowing the use of a narrow vertical web connecting the flanges. Thus for a given load supporting capacity, an I-beam is lighter than a solid beam having a rectangular cross section.
Although metal I-beams have long been used, within their strength range wooden I-beams are often superior. They cost less, are more easily cut to fit, and since they don't require special fasteners, are better adapted to wood frame construction such as houses and small office buildings.
Typically, to economically manufacture such beams having reliably consistent and uniform strength, clear, straight-grained lumber is used for the flange members, and the web is made from plywood and glued to both flanges.
Various adhesive-joint arrangements have been used in prior art wooden I-beams with some success, but with the disadvantage in some designs that assembly of the beam produces residual stresses in the composite beam member. In other designs, the sizable amount of wood cut from flange members, in the making of the joint, tends to weaken the flanges, reducing overall beam strength.
In one prior design, disclosed in Troutner U.S. Pat. No. 3,490,188, opposite margins of the web members are pressed to a tapered shape and adhesively fastened in a tapered groove in each flange. As the web absorbs adhesive, when it is inserted into the groove of the flange, the wood therein swells and thus the web tends to resume its original shape. The swelling causes a residual stress in the completed I-beam tending to split the flange.
Another prior means of connecting the web to the flanges of a wooden I-beam involves forcing a web member, having two thin parallel ribs on its edge, into divering grooves in the flange members, thus creating residual stresses tending to separate the laminations of the plywood web.